Typically, a boiler is comprised of a steam side and a combustion gas side. As fuel is combusted, steam flowing through the boiler tubes is heated by the combustion gases for service use, such as driving a steam turbine. Conventional boiler-steam turbine systems also comprise superheater, reheater and economizer sections to fully use the heat of the combustion gases. Thus, the combustion gases travel over several heat transfer surfaces before discharging through the furnace stack.
The combustion of coal in a boiler furnace generates large quantities of ash and soot in the combustion gases, a portion of which deposits on the heat transfer surfaces of the furnace system. As the layer of soot builds, the heat transfer rate from the hot gas to the steam side is reduced. To remove the soot a jet of air or steam is directed by a sootblower to clean a local area. Many sootblowers are installed throughout a furnace. Although the fouling of the furnace is a gradual process, sootblowing results in an abrupt change in local heat transfer and in the distribution of energy to the steam side components in a furnace. The former can cause transients in steam temperature as the control system responds to the disturbance. The latter can cause a loss of performance by driving steam temperatures below the range of control and reducing the achievable steam temperatures thereby resulting in an unfavorable energy distribution that can lead to a loss of thermal performance. Also, since sootblowers use pressurized air or steam, excessive sootblowing wastes energy. Further, the activation of certain sootblowers that have little effect on performance results in ineffective use of energy.
Large numbers of sootblowers and continuous changing unit conditions make it difficult for an operator to determine which sootblower to activate and when. Operators typically have resorted to a time basis for the selection of sootblower operation. As a result, unfavorable furnace energy distributions frequently develop, which leads to a loss of performance. One indication of poor distribution is the inability to achieve temperature set point with reheat steam. Reheat steam temperature controls simply are not able to raise steam temperatures to the set point due to excessive fouling of the reheat sections.
Additionally, an abrupt change in heat transfer due to sootblowing often results in large transients in steam temperatures that challenge a steam temperature control system. A conventional steam temperature control system does not anticipate the impact of sootblowing on steam temperature. Therefore, the control system relies solely on feedback action to respond to the effect of sootblowing. This results in significant deviations from set point for several temperature cycles as steam temperatures gradually return to set point or to the limit of the control range.
To help decrease these changes, recently many power plants have begun to upgrade and retrofit their locations to improve the controls. Performance history has established that poor control is a primary contributor to plant trips and premature equipment failures. To gain more control, many utilities have installed a distributed control system (DCS). However, an upgrade to new control hardware alone does not resolve the plant control problems. Control logic must also be used to achieve and enhance control improvement especially in light of the complexity of modern power plants, the highly interactive nature of the process and the large number of controllable parameters.